Some catalyst components are useful for the stereospecific polymerization of olefins. In some instances, the polymerization of propylene uses Ziegler-Natta catalysts made from or containing a solid catalyst component, constituted by a magnesium dihalide on which are supported a titanium compound and an internal electron donor compound, used in combination with an Al-alkyl compound. In some instances, the preparation of higher crystallinity of the polymer also involves the use of an external donor, thereby obtaining higher isotacticity. An alkoxysilane can act as an external donor. In some instances, esters of phthalic acid are used as internal donors in catalyst preparations. In some instances, phthalates are used as internal donors in combination with alkylalkoxysilanes as external donor. This catalyst system gives good performances in terms of activity, isotacticity and xylene insolubility.